I have an online teaching job here in the States. I'm considering TEFLing in Thailand and want to keep my online job while I do it. I'm wondering, are there any visa restrictions against working a non-Thailand based job while working a Thailand-based job? Also, has anyone seen employers restrict their full time teachers from having a second job?

It's under the radar so it is a non-issue unless you plan to do it at work.

Your major problem will be your internet speed.
Typically here, download speeds are decent (ADSL and 4-12mbps) but your upload speed will be slow (500kbps - 1mbps) if you plan to do much video conferencing.

I won't need super speedy internet. Actually, I'm more concerned with having a secure connection. Do you know anything about the security of public Wifi there? The website I use itself is secure so in theory it's fine to use public wifi. But I like to be as far on the safe side as possible. And is it possible to get internet in your apartment? I assume that would be more secure than a public connection.

So, no visa restrictions on having a second job, correct? And it's not common for work contracts to forbid a 2nd job?

I won't need super speedy internet. Actually, I'm more concerned with having a secure connection. Do you know anything about the security of public Wifi there? The website I use itself is secure so in theory it's fine to use public wifi. But I like to be as far on the safe side as possible. And is it possible to get internet in your apartment? I assume that would be more secure than a public connection.

So, no visa restrictions on having a second job, correct? And it's not common for work contracts to forbid a 2nd job?

Thanks a lot for your help on this...

If you have a non-b visa and work permit then anything (work) NOT included in your W/P is illegal.
If you are on a tourist visa then any work (paid or not) is illegal.

Working on-line with your pay going into a non-Thai account flies under the radar and enforcement is "0".

Yes, you can get ADSL in your apartment.
Yes, response times are long (long ping times) so the lag (delay) is more.
In an internet cafe or wifi at McDs the lag it may make a video call untenable or impossible.